Glycoproteins are one of the integral components of eukaryotic plasma membranes. The sugars are added to the proteins in one of two ways - either by stepwise transfer from a sugar-nucleotide donor or by a concerted transfer, having been preassembled into an oligosaccharide on a phosphoisoprenol lipid. All of the discrete reactions involved in the assembly of the oligosaccharide-lipid and in the transfer of that oligosaccharide to the protein are not understood. Our approach to this problem has been to isolate and characterize mammalian cell mutants which are defective in glycosylation reactions involving the lipid intermediates. We are currently characterizing concanavalin A-resistant Chinese hamster ovary cells which synthesize low levels of oligosaccharide-lipids.